Construction method for fault zone lower adit small-angle oblique crossing large-span cavern

Abstract

The invention discloses a fault zone lower adit small-angle oblique crossing large-span cavern construction method which comprises the following steps that S1, an adit is excavated, and the adit is excavated along a small-angle line till the end of the adit completely enters the range of a main cave; s2, the upper portion of the main channel is excavated, and excavation is conducted from the working face of the upper portion of the main hole to the variable section of the main hole; s3, the remaining upper area of the main hole is excavated, and advanced reinforcement is adopted for the fault zone; s4, deslagging is conducted through the branch holes, and the lower areas of the main holes are excavated from the branch holes to the two ends; and S5, a lower secondary lining of the section section is constructed, then concrete is integrally poured into a curved-section ring beam at the position of the intersecting line of the secondary lining of the intersecting section and the cavern, a permanent supporting structure of the intersecting section of the branch cavern and the main cavern is formed, and finally the remaining secondary lining is constructed. According to the method, the safety of the construction process when the branch hole enters the large-span cavern at a small angle can be guaranteed, the influence on the stability of the main hole is reduced, meanwhile, the construction sequence and the supporting mode are reasonably arranged, and the construction efficiency is improved.

Description

Technical Field

[0001] The invention relates to the technical field of underground tunnel construction, and in particular to a method for constructing a branch tunnel with a small angle and a large span under a fault zone. Background Art

[0002] When excavating underground caverns in water conservancy projects, there are often two difficulties when a fault zone exists at the intersection of the caverns and the branch tunnels need to be connected to the long-span main tunnel at a small angle (≤25°): first, the excavation of the large-span cavern at the small angle section is prone to instability accidents, and second, it is difficult to ensure the safety of the excavation of the intersection section of the caverns under the fault zone. Currently, the excavation methods for the sudden change from a small-section cavern to a large-section cavern include: small pilot pit climbing and expansion method, cross-channel large package method, CRD method conversion construction, and uplift hole method conversion construction. These methods generally have the following shortcomings during construction:

[0003] 1) During section conversion, there is insufficient construction space and large machinery cannot enter the hole, which affects the construction speed and prolongs the construction period;

[0004] 2) The amount of temporary support is large and the removal is frequent, which increases costs and affects the construction speed;

[0005] 3) The arch is expanded more times and takes shape more slowly, which is not conducive to early closure of the lining.

[0006] Therefore, it is necessary to develop reliable excavation technology for small-section caverns with oblique mutation to large-section caverns to address the shortcomings of existing technologies. Summary of the Invention

[0007] The purpose of the present invention is to address the deficiencies in the above-mentioned existing technologies and to provide a method for constructing a large-span cavern with a small-angle oblique branch tunnel under a fault zone. This method can ensure the safety of the construction process when the branch tunnel enters the large-span cavern (hereinafter referred to as the main tunnel) at a small angle, reduce the impact on the stability of the main tunnel, and at the same time reasonably arrange the construction sequence and support method to improve construction efficiency.

[0008] The technical solution adopted by the present invention to solve the technical problem is: a method for constructing a small-angle oblique large-span cavern under a fault zone, the steps of which are as follows:

[0009] S1. Excavate the branch tunnel along a shallow angle until its end completely enters the main tunnel. During excavation, primary support, anchor bolts, and secondary lining are installed in the branch tunnel area for reinforcement. Only primary support reinforcement is installed in the main tunnel area. After the secondary lining of the branch tunnel is completed, its structure will absorb the stress concentration on the angled side of the long-span cavern excavation, ensuring construction safety.

[0010] S2. Excavate the upper portion of the main tunnel, starting from the upper working surface of the main tunnel and proceeding in an angled direction until the main tunnel cross-section changes. After excavation is complete, install primary support to create space for the construction of the intersecting support structure. Install embedded arches and pipe scaffolding reinforcement toward the intersecting section to form the top support structure of the intersecting excavation surface, ensuring stability during subsequent excavation of the main tunnel.

[0011] S3. Excavate the remaining upper area of ​​the main tunnel. The fracture zone will be reinforced using a combination of advanced pre-grouting and advanced small pipes. Curtain grouting will be used to plug leaks. The fracture zone location will be determined based on the results of the advanced geological forecast. In the overlapping area between the main tunnel and branch tunnels, when the initial support of the branch tunnel is encountered during excavation, gradual blasting will be used. The blasting advance will be 1m per steel arch spacing. The intruding steel sections of the branch tunnel will be promptly cut, and earth will be filled in the branch tunnel to form a slope support to ensure the stability of the construction platform. The fracture zone will be reinforced using a combination of advanced pre-grouting and advanced small pipes. The sudden water inrush section will be reinforced using advanced pre-grouting. After excavation is completed, initial support and anchor bolting will be carried out in the upper area of ​​the main tunnel. For small-angle sections, anchor bolting will be installed at a small angle.

[0012] S4. Remove slag through the branch tunnels and excavate the lower area of ​​the main tunnel. Excavate the lower area of ​​the main tunnel from the branch tunnels toward both ends. After excavation is completed, install primary support and anchor bolts in the lower area of ​​the main tunnel, and reinforce the angled sections with short anchor bolts.

[0013] S5. After completing all the primary supports of the main tunnel, first complete the construction of the lower secondary lining of the high-section section, then pour concrete into the curved cross-section ring beam at the intersection line of the secondary lining of the intersection section and the cavern to form a permanent support structure for the intersection section of the branch tunnel and the main tunnel, ensure the stability of the structure at the intersection and the safety of subsequent construction, and then construct the remaining secondary lining.

[0014] Compared with the existing technology, the beneficial effects of the present invention are: this design is different from the traditional excavation method. This method uses a branch tunnel to intersect the main tunnel at a small angle, and the second lining of the branch tunnel on one side of the angle bears the excavation stress of the large-span cavern. The variable section design and embedded sleeve arch are used to ensure the stability of the excavation of the intersection section, and the construction work surface is more efficient. The curved section ring beam is used to bear the stress at the intersection of the caverns, ensuring the safety and stability of the structure and construction safety. BRIEF DESCRIPTION OF THE DRAWINGS

[0015] Figure 1 A flowchart of the steps of an embodiment of the present invention;

[0016] Figure 2 A three-dimensional schematic diagram of an oblique underground cavern provided by an embodiment of the present invention;

[0017] Figure 3 A schematic diagram of an oblique plane of an underground cavern provided by an embodiment of the present invention;

[0018] Figure 4a-4d It is a schematic diagram of the excavation sequence of the present invention;

[0019] Figure 5 This is a schematic cross-sectional view of the main hole of the present invention;

[0020] Figure 6 It is a schematic longitudinal section diagram of the sleeve arch at the variable section of the present invention;

[0021] Figure 7 It is a schematic cross-sectional view of the sleeve arch at the variable section of the present invention;

[0022] Figure 8 It is a schematic cross-sectional diagram of the intersecting section excavation of the present invention;

[0023] Figure 9 Schematic diagram of the cross section of the curved cross-section ring beam of the present invention;

[0024] Figure 10 and Figure 11 These are stress simulation test diagrams for the traditional process and the present invention.

[0025] In the figure, 11-intersection section of branch tunnel and main tunnel, 12-secondary lining of branch tunnel, 13-steel arch frame of branch tunnel, 14-slope support, 21-steel arch frame of main tunnel, 22-secondary lining of main tunnel, 23-advanced small guide tube, 24-grouting anchor of main tunnel, 25-locking anchor of main tunnel, 27-embedded sleeve arch, 28-pipe shed, 29-variable section of main tunnel, 3-curved section ring beam, 4-angle section, 41-short anchor. DETAILED DESCRIPTION

[0026] The present invention will be further described below with reference to the accompanying drawings and specific embodiments. It should be noted that the following specific embodiments are only representative embodiments of the present invention, wherein the specific methods, devices, conditions, materials, etc. exemplified are not intended to limit the present invention or the corresponding specific embodiments.

[0027] A method for constructing a large-span tunnel with a small-angle oblique cross section under a fault zone, such as Figure 1-9 As shown, the steps are as follows:

[0028] S1. Figure 4a-4dAs shown, ① excavation of the branch tunnel is performed along a shallow angle until the branch tunnel end completely enters the main tunnel. During excavation, primary support, anchor bolts, and secondary lining are installed in the branch tunnel area for reinforcement, while only primary support reinforcement is installed in the main tunnel area. In this embodiment, the branch tunnel excavation method adopts a step method, with two layers of excavation at a height of 3-6m. After each layer of excavation, hollow grouting anchor bolts and steel arch frames 13 are installed. The grouting anchor bolts are 4.5m long and arranged in a plum blossom pattern with a spacing of 1.0m between rows. The steel arch frame 13 is used as the primary support reinforcement. The longitudinal spacing of the steel arch frame 13 is 1000m, the longitudinal connecting reinforcement is φ22, and the circumferential spacing is 1000mm. In Class III surrounding rock, the steel arch frame steel can be omitted as the steel frame. The secondary lining 12 is applied in the branch tunnel area, and 0.5m thick C35 cast-in-place concrete is used to form an arch structure. The structure bears the stress concentration on the angle side of the large-span cavern excavation to ensure the safety of subsequent construction. Only the primary support is carried out in the intersection section 11 between the branch tunnel and the main tunnel.

[0029] S2.② Excavate the upper portion of the main tunnel, starting from the upper working surface of the main tunnel and proceeding in an angled direction until the main tunnel cross-section 29 is reached. After excavation is complete, a steel arch frame 21 and shotcrete are installed as primary support to create space for the support structure of the intersecting section. Subsequently, an embedded sleeve arch 27 is constructed toward the intersecting section. The embedded sleeve arch 27 is fully embedded in the top of the small cross-section, providing end stability for the pipe support 28. The pipe support 28 is reinforced and forms a support structure to ensure safe excavation at the main tunnel intersection. An I-beam is installed within the embedded sleeve arch 27 as the steel arch frame 21. A Φ127*8mm guide steel pipe is embedded in the outer edge of the 120° steel frame at the top of the embedded sleeve arch 27. Each guide pipe section is 5m long and has an outward inclination angle of 1° to 3°. The steel bars are welded to the guide pipe and the I20a I-beam frame on both sides. The embedded sleeve arch 27 is constructed with cast-in-place concrete, 0.6m thick, and the concrete is poured in a single step. Drilling can begin after the arch concrete reaches 75% strength. The larger pipe is the pipe-roof grouting steel pipe, with a diameter of 127mm. A 108mm diameter, 10mm wall thickness, and a length of 25m are used. Grouting is performed through the reserved grouting pipe, and the reinforcement of the pipe-roof 28 covers the top 1-3m of the intersection. After grouting, the grouting is promptly removed from the pipe and tightly filled with M30 cement mortar to enhance the rigidity of the steel pipe. Once the grouting strength reaches 15MPa, forming the pipe-roof 28 reinforcement body, the next construction step can be carried out.

[0030] S3.③ Excavate the remaining upper area of ​​the main tunnel to a height of 5-6m. A gentle slope with a slope ratio of 1:8 will be constructed downward from the constructed area, gradually transitioning to the variable cross-section area. Before construction, 23 φ42 small pre-drilling pipes will be installed in the arch. These pipes are 4.5m long, spaced 0.4m circumferentially, and 3m longitudinally. The fault zone will be reinforced using a combination of pre-grouting and pre-drilling pipes. Leakage will be sealed using curtain grouting. The fault zone location will be determined based on the results of the advanced geological forecast. A new pipe shed was constructed approximately three meters from the edge of the pipe shed reinforcement. In the overlapping area between the main tunnel and branch tunnel, when excavation encountered the steel arch 13 of the branch tunnel's primary support, the top rock layer of the intersection was gradually blasted. The blasting interval was 1 meter for each branch tunnel steel arch, and one branch tunnel arch was destroyed with each cycle. The intruding steel of the branch tunnel was promptly cut, and the excavated soil and rock were used to fill the branch tunnel, forming a 1:2 slope support 14 to ensure the stability of the construction platform at the intersection. The fracture zone was reinforced using a combination of advanced pre-grouting and advanced small pipes; the sudden water inrush section was reinforced using advanced pre-grouting. After the upper section was fully excavated, 7.5m long Φ28 hollow grouting anchors were installed in the arch of the upper area of ​​the main tunnel, with a spacing of 1.25 mm. 24 Φ28 hollow grouting anchors (L = 6m) were installed on the upper sidewalls of the main tunnel, using a 20mm steel frame with a longitudinal spacing of 1000 mm. 25 Φ25 mortar-locked anchors were also installed. For small corner sections, the anchors were installed with oblique anchors at small angles.

[0031] S4.④ Begin excavation at the branch tunnel to expand the lower area of ​​the main tunnel. Starting from the slope support 14, begin excavating the lower half of the main tunnel. Using a step method, excavate in two layers, with a height of 4-6m. Deslagging will be carried out through the branch tunnel. After excavation is complete, excavate the lower area of ​​the main tunnel from the branch tunnel toward both ends. ⑤ Install primary support and Φ28 hollow grouting anchors 24 (length L = 4.5m, spacing 1.25m) and Φ25 mortar-locking anchors 25 at the lower portion of the sidewalls in the lower area of ​​the main tunnel. When excavating the angled section, reinforce the angled section 4 with short anchors 41. Short anchors 41 range in length from 1.8 to 4.5m and are spaced from small to large along the angle.

[0032] S5. After completing the overall primary support of the main tunnel, first construct the lower secondary lining of the high-section section. Then, construct the curved-section ring beam 3 of the portal according to the outer contour of the intersection of the branch tunnel and the main tunnel. The steel bars of the ring beam 3 and the steel bars of the secondary lining 22 of the main tunnel are tied simultaneously. The secondary lining steel bars of the branch tunnel and the main tunnel are both anchored into the ring beam with an anchoring length of 35d. After the steel bars are tied, adjust the formwork trolley so that the ring beam and the wall are cast as a whole, forming a permanent support structure for the intersection of the branch tunnel and the main tunnel, ensuring the stability of the structure at the intersection and the safety of subsequent construction. Then, construct the remaining secondary lining.

[0033] This solution is suitable for underground caverns with fault zones, where the main tunnel is excavated with branch tunnels at a small angle. By constructing a secondary lining in the branch tunnel on one side of the angle, improving the excavation sequence, and constructing variable-section embedded arches, and other technical methods, the difficult engineering problems of the traditional excavation method, such as long construction time, small construction space, and unstable support effect, are solved. This solution is simple to operate. The secondary lining of the branch tunnel is constructed first, so that the secondary lining structure bears stress concentration on the angle side between the branch tunnel and the main tunnel. The arch is constructed at the variable section to ensure the construction working space of the intersecting section of the excavation, and the excavation sequence is adjusted to improve construction efficiency. The overall structural stress is stable, so that no additional reinforcement measures are required at small angles. This solution adopts an ideal construction method to shorten the construction period and ensure safety, while saving a lot of material costs, machinery and equipment costs, labor costs, and construction period costs for the project. Stress simulation was performed using the geotechnical finite element analysis software MIDAS GTS NX. The software simulated the impact of the traditional process and the excavation of this solution on the angle section, such as Figure 10 As shown in Figure 2, the traditional process of excavating branch tunnels from the main tunnel has a stress peak of 5800. Figure 11 As shown in the figure, this scheme adopts excavation from the branch tunnel into the main tunnel, and its stress peak is 2300. It can be found that the stress concentration in the angle section of this scheme is effectively controlled, which is only half of that of the traditional process, and the stress is evenly controlled in the angle section.

[0034] The above description is only a preferred embodiment of the present invention and should not be used to limit the scope of implementation of the present invention. In other words, any simple equivalent changes and modifications made according to the scope of the patent application and the content of the invention description are still within the scope of the patent of the present invention.

Claims

1. A method for constructing a small-angle oblique large-span tunnel under a fault zone, characterized in that: The steps are as follows: S1. Excavate a branch tunnel along a shallow angle until the end of the branch tunnel completely enters the main tunnel. S2. Excavate the upper part of the main channel, starting from the upper working surface of the main tunnel in the direction of the included angle, until the main tunnel cross-section changes; S3. Excavate the remaining upper area of ​​the main tunnel and use advance reinforcement on the fault zone; S4. Remove slag through the branch tunnel and excavate the lower area of ​​the main tunnel from the branch tunnel toward both ends; S5. Construct the lower secondary lining of the cross-section, then pour concrete into the curved ring beam at the intersection of the secondary lining and the cavern to form a permanent support structure for the intersection of the branch tunnel and the main tunnel. Finally, construct the remaining secondary lining.

2. The method for constructing a small-angle oblique large-span tunnel under a fault zone according to claim 1 is characterized in that: In step S1, during excavation construction, primary support, anchor rods and secondary lining reinforcement are applied in the branch tunnel area, and only primary support reinforcement is applied in the main tunnel area.

3. The method for constructing a small-angle oblique large-span tunnel under a fault zone according to claim 1 is characterized in that: In step S2, after the excavation is completed, the primary support is constructed, and the embedded sleeve arch and pipe shed reinforcement are constructed in the direction of the intersecting section.

4. The method for constructing a small-angle oblique large-span tunnel under a fault zone according to claim 1 is characterized in that: In step S3, the fault zone is reinforced by combining advance pre-grouting treatment with advance small conduits, and leaks are blocked by curtain seepage grouting treatment.

5. The method for constructing a small-angle oblique large-span tunnel under a fault zone according to claim 1 is characterized in that: For the overlapping area of ​​the main tunnel and the branch tunnel, when excavation encounters the initial support of the branch tunnel, gradual blasting construction is adopted. The blasting advance is 1m for the spacing of the steel arch frame of the branch tunnel, and backfill is carried out at the branch tunnel to form an earth slope support.

6. The method for constructing a small-angle oblique large-span tunnel under a fault zone according to claim 1 is characterized in that: In step S3, after the excavation is completed, primary support and anchor bolt construction are carried out in the upper area of ​​the main hole.

7. The method for constructing a small-angle oblique large-span tunnel under a fault zone according to claim 6 is characterized in that: When constructing anchor rods, use inclined anchor rods at small angles.

8. The method for constructing a small-angle oblique large-span tunnel under a fault zone according to claim 1 is characterized in that: In step S4, after the excavation is completed, primary support and anchor rods are applied in the lower area of ​​the main tunnel, and short anchor rods are applied to reinforce the angle section.

9. The method for constructing a small-angle oblique large-span tunnel under a fault zone according to claim 1 is characterized in that: In step S5, all primary supports of the main tunnel are completed first, and then the lower secondary lining of the section is constructed.

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